GEOLOGICAL SURVEY OF THE TERRITORIES. 213 



reception of tlie numerous little mountain rivulets that flow down from 

 the surrounding rim, collecting them together and discharging them at 

 one outlet. Thus the North Park collects the various streams which 

 form the IsTorth Platte ; the Middle Park, those that form Blue Eiver ; 

 South Park, those to form the South Platte ; the San Luis Park, those 

 to form the Eio Grande ; and the Upper Arkansas Yalley, which is a 

 true park, those to form the Arkansas Eiver. Here, then, we see that 

 five of the great rivers of this vast central region have their sources 

 close together in this mountain area. Upon the peaks, ranges, parks, 

 and forests embraced between the one hundred and fifth and one hun- 

 dred and seventh meridians and thirty-eighth and forty -first parallels, 

 an area not exceeding eighteen thousand square miles, depend, in a 

 great measure, the agricultural resources of an area of more than one 

 hundred thousand square miles. 



Before passing over to the west side of the section under consideration, 

 I would call attention to the Black Hills, (or Laramie Eange,) of Wyo- 

 ming, which seems to be the real northern extension of the Colorado 

 Eange, but the continuity is somewhat broken at the gorge of the Cache- 

 a-la-Poudre, and it takes the form of a huge appendage, like the claw 

 of a crab. Circling round the eastern and northern portions of the great 

 Laramie Park, it acts as a bracing wall to this vast elevated plain, 

 whose surface is fully 1,500 feet above the plains at the eastern base. 

 Its external, or eastern slope, presenting a much longer descent than its 

 inner or western face, differs considerably in character from the latter; 

 while the latter, at least as far north as the gorge of the Laramie Eiver, 

 presents comparatively smooth and rounded surfaces, the former is rugged, 

 and, especially along the northern part, deeply gashed by rough and rocky 

 caiions. The intervening ridges are quite rugged up in the mountain 

 near their origin, but as they descend to the plain they gradually lose 

 their rough character, and grow smoother and rounder, and, seen trans- 

 versely, present a succession of rounded foot-hills, which appear like the 

 waves of the sea. The eastern flank and summit are tolerably well 

 wooded, and the northern portion appears to have a timber growth pretty 

 generally distributed over it, but interrupted by numerous open, field- 

 like spaces. Numerous small streams that form tributaries to the North 

 Platte have their origin on the eastern slope, while on the west but one 

 or two have their sources in this range. 



Between the eastern and western portions of this mountain group 

 intervenes a broad but irregular depression, forming the Green Eiver 

 basin. The broad, elevated plain, formerly called the Colorado Desert, 

 which stretches north and south from the Wind Eiver Eange to the Uintah. 

 Mountains, and east and west from the Wahsatch Eange to the imper- 

 ceptible divide, separating it from Laramie Plains, forms the upper por- 

 tion. Having a gentle southern slope, and inclination to a central 

 channel, it collects the waters, which once evidently formed an immense 

 lake, against the mountain barrier at the south margin, of which an 

 account will be found in Professor Hayden's report for 1870. Having, 

 in the geological ijast, burst through this barrier, a tortuous channel 

 has been formed for the waters, by which they connect with the Colo- 

 rado Eiver and its vast water system farther south, receiving large 

 contributions from right and left in its passage. 



Shooting out from the Wahsatch Eange on the west, the Uintah Mount- 

 ains stretch directly eastward, forming the southern wall to the upper 

 portion of this basin, forcing Green Eiver, in making its exit from the 

 northern plains, to bend eastward in order to flank them. This range, 

 which has a direction the reverse of the general course of the mountains 



